As a typical method of noise suppression for emphasizing an intended signal, a voice signal or the like, by suppressing noise, an unintended signal, from an input signal into which noise is mixed, a spectral subtraction (SS) method has been known, for example. The SS method carries out noise suppression by subtracting from an amplitude spectrum an average noise spectrum estimated separately (see Non-Patent Document 1, for example).
When noise suppression such as a spectral subtraction method has been performed, estimated errors of the noise spectrum remain in the signal after the noise suppression as distortions which give characteristics very different from the signal before the processing and appear as harsh noise (also called artificial noise or musical tone), thereby sometimes deteriorating subjective quality of the output signal greatly.
As a method of suppressing the subjective deterioration feeling mentioned above, there is one disclosed in Patent Document 1. Patent Document 1 aims at providing a noise suppressor that does not produce musical noise in noise intervals, and does not produce distortion in voice intervals. It comprises a voice/noise decision unit for deciding intended signal intervals and noise signal intervals from the input signal; a noise suppressing unit for suppressing noise from the input signal and estimated noise signal in accordance with a first suppression coefficient; a noise over-suppressing unit for suppressing noise from the input signal and estimated noise signal in accordance with a second suppression coefficient greater than the first suppression coefficient; and a switching unit for switching between the output signal of the noise suppressing unit and the output signal of the noise over-suppressing unit in accordance with the decision result of the voice/noise decision unit.    Non-Patent Document 1: Steven F. Boll, “Suppression of Acoustic noise in speech using spectral subtraction”, IEEE Trans. ASSP, Vol. ASSP-27, No. 2, April 1979.    Patent Document 1: Japanese Patent Laid-Open No. 2005-195955 (pp. 8-9, and FIG. 1 and FIG. 2).
With the foregoing configuration, the conventional noise suppressor switches between the output signal of the noise suppressing unit and the output signal of the noise over-suppressing unit in accordance with the decision result of the voice/noise decision unit. Accordingly, it has a problem of being unable to avoid quality deterioration due to erroneous decision. In addition, it has a problem of being difficult to make a completely correct decision because the voice signal and noise signal are infinitely various and involves time fluctuations.
In particular, if it makes an erroneous decision that a noise signal interval is a voice signal interval, it produces musical noise in that interval, thereby offering a problem of greatly deteriorating the quality.
In addition, even in voice signal intervals, if voice components are very small when considered from the individual frequency bands, a problem arises in that if there is a band in which the noise components are dominant, musical noise arises in that band, thereby deteriorating the quality greatly.
Furthermore, when it makes an erroneous decision that a voice signal interval is a noise signal interval, although it reduces the suppression of the voice by adding the input signal, if it makes erroneous decisions frequently within the same voice signal interval, a problem arises of giving a feeling of unstable fluctuations, thereby deteriorating the quality.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide a noise suppressor with high sound quality capable of reducing the occurrence of musical noise.